1. Field of the Invention
The present invention relates to a solder alloy used mainly in mounting an electronic part to a printed wiring substrate and a lead-free solder paste using the same.
2. Description of the Related Art
A solder alloy, which is melted at a low temperature and has good electrical connection characteristics, is generally used in mounting an electronic part to a printed wiring substrate.
The solder alloy is required to be excellent in each of the tensile strength, the elongation, the mechanical characteristics such as a thermal fatigue strength, the solder wettability, and the solderability.
The solder alloys include, for example, a tin-lead alloy, a tin-silver alloy, a tin-antimony alloy, and a tin-bismuth alloy. Among these alloys, the tin-lead alloy was widely used as a solder alloy excellent in the characteristics given above.
In recent years, the effects of lead on the human body have come to be considered a serious health problem. Therefore, it is very important to develop a lead-free solder alloy excellent in the characteristics referred to above.
However, a serious problem remains unsolved in the lead-free solder alloy that the melting point of the lead-free solder alloy is generally higher than that of the tin-lead alloy by at least 40° C.
A tin-bismuth binary solder alloy containing 25 to 55% by weight of bismuth and the balance of tin and unavoidable impurities is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-141079. Among the lead-free solder alloys, the tin-bismuth binary solder alloy has a relatively low melting point. However, it was difficult for the tin-bismuth binary solder alloy to simultaneously satisfy the mechanical characteristics and the wettability, with the result that tin-bismuth binary solder alloy is insufficient for use as a substitute for tin-lead solder alloy.
Under the circumstances, it is very important to develop a lead-free solder having a low melting point, and being capable of sufficiently satisfying the wettability, the solderability, and various mechanical characteristics.